1. Field of the Invention
The invention relates to a method and to a device for improving the overall spectral efficiency of wireless telecommunication systems.
2. Description of the Related Art
As known by a person skilled in the art, several techniques have been proposed and partially implemented to enhance the spectral efficiency of systems operating with the GSM-standard (Global Systems for Mobile Communication). Techniques for enhancing the spectral efficiency are especially needed for operators, which suffer from limited capacities in particular due to a limited spectrum licenses, that is often the case for GSM operators, in particular in the USA. It is specific to GSM in contrast to other mobile standards, that there is the need to dedicate a big portion of the operator's spectrum for beacon purposes, which includes for example signalling, paging and synchronisation.
Even if there is only one beacon frequency for respective one base transmitter station (BTS), the spectrum is wide due to a high cluster size of typically 15 . . . 18 cells. Considering an operator having a license for 5 MHz, this means that 3 to 3.6 MHz of the operator's spectrum is given away for a single frequency, which accordingly is covering about 60 to 72% of the total spectrum. In contrast thereto, a typical cluster size for traffic frequencies is about ⅓ of the beacon cluster size, i.e. around 5 to 6 cells.
In order to improve the overall total spectral efficiency different techniques are known and may be applied.
One of these known techniques comprises the use of adaptive arrays. However, systems incorporating such adaptive arrays are very complex. They need a lot of DSP (Digital Signal Processor) processing power and a huge effort with the radio frequency processing. Moreover, multi-carrier amplifiers are sometimes needed which are difficult, in particular concerning their designing for GSM because of high linearity requirements. Summarising, the costs are very high and such a system is hard to be managed and to be planned. Additionally the adaptive arrays only enhance the spectral efficiency of the traffic portion of the operator's spectrum but do not improve the spectral efficiency with the portion of the spectrum, which is used for the broadcast control channel (BCCH). Consequently, when using a system incorporating adaptive arrays still a lot of spectrum is wasted for beacon purposes, i.e. for example for the broadcast channel.
A further known technique utilises the compact mode, formally the enhanced data rate through GSM evolution (EDGE) compact. Even if the improvement by running compact mode and also the effort for its implementation is moderate, one of the problems with compact is, that it implies a totally new signalling scheme, which existing mobiles are not able to decode. Accordingly, this is not a preferred solution for established networks or those who want to offer roaming for foreign users.
Still another known technique proposes the use of transmitter diversity, i.e. TX diversity, through delay diversity enhancing the performance only for low dispersion profiles, like with urban scenarios. For rural applications with late echoes however, a high time dispersion and performance degrading is a result. A further disadvantage is that depending on the number of artificial delay paths multiple transceivers are needed causing this solution to be coastwise unattractive.
Still a further known technique consists of the use of TX diversity through antenna hopping using simple cyclic antenna hopping sequences. Antenna hopping or equivalent polarisation hopping or a combination of both does not suffer from the problem of increased hardware effort since it can be realised be means of additional software for the combination of baseband and synthesiser hopping.
Since an antenna hopping scheme also is forming a basis of the invention for the following description and the appended set of claims the term of antenna hopping is generally standing as a synonym for polarisation, too.
The problem with cyclic antenna hopping sequences, however, is that only the signalling channels but not the synchronisation channels for frequency and time synchronisation are improved, since for example the broadcast control channel is combined with other logical channels which partially are not organised in blocks of four bursts like the FCCH (Frequency Correction Channel) and the SCH (Synchronisation Channel). Accordingly, a simple cyclic antenna hopping sequence like ABABABAB with two antennas “A” and “B” or ABCDABCDABCDABCD with four antennas “A”, “B”, “C” and “D” is neither a good solution.